In numerous processes described in the patent literature, for example U.S. Pat. Nos. 3,626,020 to Neuzil, 3,663,638 to Neuzil, 3,665,046 to deRosset, 3,668,266 to Chen et al., 3,686,342 to Neuzil et al., 3,700,744 to Berger et al., 3,734,974 to Neuzil, 3,894,109 to Rosback, 3,997,620 to Neuzil and 4,014,949 to Hedge, particular zeolitic adsorbents are used to separate the para isomer of dialkyl substituted monocyclic aromatics from the other isomers, particularly para-xylene from other xylene isomers. Many of the above patents use benzene, toluene, or p-diethylbenzene as the desorbent. P-diethylbenzene (p-DEB) has become a commercial standard for this separation. However, p-DEB is a "heavy" desorbent (higher boiling than p-xylene) which suffers in the process for separating p-xylene from feed mixtures containing C.sub.9 aromatics because the boiling point of p-DEB is too close to the boiling point of C.sub.9 aromatics in the feed. Because the C.sub.9 aromatics are difficult to separate from p-DEB by simple fractionation, the C.sub.9 aromatics would gradually build up in the desorbent, which must be recycled for economic reasons. In the commercial process for recovering p-xylene from feed mixtures containing isomers, therefore, it has been necessary to reduce C.sub.9 aromatics in the feed to below about 0.1% prior to the adsorptive separation of p-xylenes. This is usually done by distillation in a so-called xylene splitter column. Of course, substantial costs associated with this practice, such as capital costs of the xylene splitter and utilities necessary to achieve substantially complete removal of the C.sub.9 aromatics, could be reduced greatly or eliminated if it were not necessary to first remove C.sub.9 aromatics. Thus, while U.S. Pat. No. 3,686,342, supra, mentions tetralin as a possible heavy desorbent for the para-xylene separation process, that reference clearly states that p-DEB is the best desorbent for the separation and, further, does not address the problem that the preferred desorbents may have in separating feeds containing C.sub.9 aromatics. Therefore, a higher boiling point material, that meets the selectivity requirements for desorbents and can be separated from C.sub.9 aromatics, has been long sought and is still desirable.
It is also known that crystalline aluminosilicates or zeolites are used in adsorption separations of various mixtures in the form of agglomerates having high physical strength and attrition resistance. Methods for forming the crystalline powders into such agglomerates include the addition of an inorganic binder, generally a clay comprising a silicon dioxide and aluminum oxide, to the high purity zeolite powder in wet mixture. The blended clay zeolite mixture is extruded into cylindrical type pellets or formed into beads which are subsequently calcined in order to convert the clay to an amorphous binder of considerable mechanical strength. As binders, clays of the kaolin type, water permeable organic polymers or silica are generally used.
The invention herein can be practiced in fixed or moving adsorbent bed systems, but the preferred system for this separation is a countercurrent simulated moving bed system, such as described in Broughton U.S. Pat. No. 2,985,589, incorporated herein by reference. Cyclic advancement of the input and output streams can be accomplished by a manifolding system, which are also known, e.g., by rotary disc valves shown in U.S. Pat. Nos. 3,040,777 and 3,422,848. Equipment utilizing these principles are familiar, in sizes ranging from pilot plant scale (deRosset U.S. Pat. No. 3,706,812) to commercial scale in flow rates from a few cc per hour to many thousands of gallons per hour.
The invention may also be practiced in a cocurrent, pulsed batch process, like that described in U.S. Pat. No. 4,159,284 or in a cocurrent, pulsed continuous process, like that disclosed in Gerhold U.S. Pat. Nos. 4,402,832 and 4,478,721.
Also, in some cases illustrated herein, it is necessary to remove three product streams in order to obtain a desired product intermediate in adsorption strength from an extract and a raffinate stream. This intermediate stream can be termed a second raffinate stream, as in U.S. Pat. No. 4,313,015 or a second extract stream, as in U.S. Pat. No. 3,723,302, both incorporated herein by reference, the latter incorporating abandoned application Serial No. 100,105 filed December 21, 1970. This case pertains when a contaminating component in the feed, such as p-ethyltoluene, is more strongly adsorbed than the desired product, p-xylene. It is not always necessary to remove p-ethyltoluene from p-xylene, e.g., where terephthalic acid is the final product of the oxidation of p-xylene, since oxidation of p-ethyltoluene results in the same product. However, if it is desired to keep the concentration of the contaminating component in the product as low as possible, a first extract is taken off, high in concentration of the desired component and lower in the contaminating product followed by a second extract, withdrawn at a point in zone 3 between the desorbent inlet and the first extract point, containing a high concentration of the contaminant and a lower concentration of the desired product. It is not necessary, however, to use a second desorbent, if the desorbent is able to first desorb the lightly held product and then desorb the remaining more strongly held contaminants, as disclosed in the aforementioned abandoned application. If the contaminating component in high concentrations and purity is desired, this can be achieved by withdrawing a second extract in the cocurrent pulsed batch process mentioned above.
The functions and properties of adsorbents and desorbents in the chromatographic separation of liquid components are well-known, but for reference thereto, Zinnen et al. U.S. Pat. No. 4,642,397 is incorporated herein.
I have discovered a process for employing a zeolite adsorbent for the separation of p-xylene from its isomers and, particularly, desorbents which are a substantial improvement in a process for separating xylene isomers where the feed mixture also contains C.sub.9 aromatic impurities. I have further discovered that the water content of the adsorbent has important effects on mass transfer rates and selectivities which may vary with the desorbent selected.